Dielectric heating with cavity resonators



Aug. 26, 1952 T. w. DAKIN ETAL 2,608,637

DIELECTRIC HEATING WITH CAVITY RESONATORS Original Filed 00:. 4, 1946 Fig. l. 6 Fi .2;

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5o .44 WITNESSES: 52 40 I INVENTORS Thomas W. Dokin, Carroll N. Wqrks Patented Aug. 26, 1952 DIELECTRIC HEATING WITH CAVITY RESONATORS Thomas W. Dakin, Pittsburgh, and Carroll N. Works, Edgewood, Pa., and Fitzhugh W. Boggs, Woodstock, N. Y., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Original application October 4, 1946, Serial No. 701,300. Divided and this application July 26, 1949, Serial No. 107,442

11 Claims. 1

Our invention broadly relates to dielectric heating apparatus; but more particularly relates to the application of cavity resonators to high-frequency dielectric heating apparatus.

An object of our invention is to provide dielectric heating apparatus including a resonant cavity, more specifically, a reentrant resonant cavity.

Another object of our invention is to provide a dielectricheating apparatus of a type described having a region with a high intensity electric field into which the material to be heat-treated can be readily placed and removed.

A more particular object of our invention is to provide a dielectric heating apparatus comprising a resonant reentrant cavity having anouter shell at ground potential and a readily accessible work-receiving space inside the outer shell. The shell protects an operator against exposure to the high-voltage parts of the equipment.

In one form of our invention, which form is shown and claimed in our application, Serial No. 701,300, filed October 4, 1946, now Patent No. 2,504,109, part of the outer shell is movable for loading the work-receiving space of the apparatus. This part must be restored in order to place theapparatus in proper operating condition. In this embodiment, the resonant cavity is provided with an inner central member having a face spaced from a parallel face of an end wall of the shell, the end wall being movable so as to permit the apparatus to be loaded and unloaded. This end wall is at a current nodal point so that the joint between it and the rest of the shell does not carry any considerable current, and'a loose joint can be tolerated, although a close fit is recommended.

In another embodiment of our invention, the resonant reentrant cavity takes the form of an injecting apparatus suitable for pre-heating or otherwise treating plastic material or the like, which is forced from the apparatus into a mold. In such an embodiment, a part of the resonant cavity is provided with an injection nose or nozzle. This embodiment, in its broad and specific aspects, forms the subject matter for claims in our present application which is a division of the aforesaid application.

Features,.innovations and objects of our invention, in addition to the foregoing, will bediscernible from the following description which is to be taken in conjunction with the accompanying schematic drawing in which:

Figure 1 is a central longitudinal view, mostly 2 in section, of apparatus embodying our invention;

Fig. 2 is a longitudinal sectional view of a modified form of our invention;

Fig. 3 is a longitudinal sectional view through an injection equipment in accordance with our invention, and

Fig. 4 is a sectional view substantiallyon the lines IVIV of Fig. 3.

In the embodiment shown in Fig. l, a resonant cavity or chamber of the reentrant type is shown. This reentrant resonant cavity is indicated in its entirety by the reference numeral 2 and comprises an outer member or shell having a hollow longitudinal tubular wall 4 and opposite end walls 6 and 8, respectively. The walls are preferably of metal so that the shell constitutes an outer metallic enclosure which can be readily grounded so as to be at ground potential. The end wall 6 is unitary with an edge of the tubular wall 4, and has secured thereto, or is integral with, a solid metallic central member [0 which coaxially extends into the tubular wall 4 and is symmetrieally arranged with respect to it. The other end wall 8 is snugly slidable inside the tubular wall 4 and can be forcibly moved to any desired distance from the central member [0 so as to con stitute a plunger. The separation between the inner face or work surface H of the end wall 8 and the facing transverse face of surface 54 of the central member l0 provides a work-receiving space in which dielectric work W, comprising a plastic or any other suitable dielectric, may be placed for heat-treatment undera pressure of a value determined by the force exerted on the outer side of the end wall 8. Thisforce is schematically indicated'at Fig. l by the series of arrows and a plunger or rod 16. In order to resist this force, the other parts of the cavity would, of course, have to be braced.

When the cavity resonator 2 is made to rescnate, a current anti-node will be at the end wall 6. A high voltage will exist between the separated faces l2 and M at the other end of the cavity resonator, and this high voltage will dielectrically heat any work between the faces.

The portions of the end wall 8 and the central member I0 between which the work is held or pressed can be considered to be relatively insulated heating electrodes, such as is found in more conventional dielectric heating apparatus. The electrode or end wall 8 can be slid into and out of the tubular wall 4 so that the dielectric heating equipment can be loaded and unloaded. However, it is obvious that the upper portion of the equipment could be moved instead, or even the central member only slid through a hole in the end wall 6. This last form, however, has the disadvantage that the sliding joint will be at a current anti-nodal point so that unless an extremely good conductive connection exists across the joint, power would be lost unnecessarily.

The shell can be made of any suitable material but preferably it should be designed so that it will absorb very little power itself. To this end, the cavity may be of steel and provided with coatings of copper or silver or other metal of high conductivity and low permeability. If desired, however, the entire resonant cavity can be made of copper.

The cavity resonator can be powered by making it part of the tank circuit of an oscillator or by coupling a conductor loop or a capacitor plate between the faces, the higher will betlie. frequency at which the resonator will oscillate.

The apparatus has an important advantage in that it provides a high degree of safety for operation because the outer) shell constitutes an enclosure which can easily be kept at ground potential. As a further precaution, any suitable safety device could be attachedto the movable wall 8 for interrupting the power to the cavity resonator whenever the wall starts to move from the heat-treating position.

The cross-section of the reentrant cavity of the resonator can be selected as'desired. The most convenient form is to make both the central member IB and the tubular member 4 circular and concentric. In such case, the end walls 6 and 8 would necessarily be round discs However, We have used a cavity resonator in which the central member and outer tubular member were square in cross-section, and had their facing sides parallel. I

As an indication of the operation of our invention, we have heated in a matter of seconds, a plastic preform 9", in diameter, 2 thick and having a loss factor'of .6 to 1, in a reentrant resonant cavity, of a type shown in Fig. l, which was square in cross-section and had the following dimensions: inside overall length 30", central member 12" square on the outside, outer tubular member 18" square on the inside. This reentrant resonant cavity operated at the frequencies from 65 me. to 80 me.

With the electrode surfaces or faces [2 and I4 flat, thevoltage between them will be greatest at the center and becomes progressively smaller between their circumference or periph ery. With flat, circular electrode surfaces, the voltage between any two longitudinally aligned points on the electrode surfaces l2 and [4 will depend on the radial distance from the axis or center. A good approximation of the voltage distribution can be obtained from the following equation:

1rer E max( where Ema): is the voltage across the two center points of the surfaces, e is the dielectric constant of the work, A is the wave length of the oscillations, and E is the voltage at the distance, r, from the center.

The amount of heating per unit volume of work varies as the square of the voltage so that the radial voltage variation may in some cases cause non-uniform heating. If the variation is large enough to be objectionable, a more uniform heating can be obtained by dishing out one of the electrode surfaces as indicated at 20 in the embodiment shown in Fig. 2, and filling the dished out portion with a solid dielectric material 22 of low power factor. The curvature of the dished out portion could be calculated for any desired voltage distribution across the work to be heated.- The voltage across any two longitudinal points at the facing heating surfaces will be partially absorbed in the filling 22, and the remainder in the work W.

Fig. 2 shows other features which have been modified from the embodiment of Fig. 1. Thus, the insulated end of the central member I8 is provided with an annular outwardly projecting portion or flange. so as to be able to heat work of larger surface or diameter. Also, the end wall 8 is hinged, by a hinge 26, to the outer tubular wall 4' and can be forcibly latched closed by a latch 28.

Teachings of our invention are also applicable to injection molding, and in Fig. 3 apparatus of this type is schematically indicated in its entirety by the, reference numeral 30. It comprises a metallic reentrant cavity having an outer tubular wall 32, an annular end wall 34 and an opposite annular transverse closing end wall 36. Extending from the inner edge of the end wall 34 is a hollow central tubular member 38. A metallic streamlined electrode member 40 is connected to the far or insulated end of the central tubular member 38 by spaced metal. connecting bars 42.

The end wall 36 is provided with an outwardly directed metallic nozzle 44 insulated from and spaced from the associated endof the central tubular member 38 by a stepped ring 46 of insulation. fThe inside of the nose 44 isprovided with a suitably shaped face or surface 48 which cooperates with a somewhat similarly shaped face or surface 50 on the electrode member 43 to form a work-receiving space across which a highfrequency voltage exists. The inside of the. central tubular member 38 is adapted to receive pellets or otherv suitable plastic material 52 which can be forced .by a ram or plunger 54 into the work-receiving space between the faces 48 and 50 of the nozzle 44 and electrodev member 49, respectively, where tlie'work is heated and finally discharged out of the nozzle or nose 44, preferably into a mold.

While we have described our invention in connection with several forms now preferred, it is obvious that the teachings thereof have wide applications and can be embodied in other radically different forms.

We claim as ourinvention:

1. Dielectric heating means comprising a reentrant resonant cavity dimensioned to resonate at a suitable predetermined frequency and comprising-an outer 'shell and an inner member, means to excite said cavity with electrical energy having said predetermined frequency, portions of said shell and said inner member being conductively secured together, and other portions of said shell having a'work surface, and said inner member having a work surface spaced from and facing the first said work surface to'form a nozzle-shaped. work-receiving space, said inner member comprising a tubular portion having a .5 hole adapted to receive work to be treated, the hole communicating with said work receiving space, and movable pressure exerting means in said hole for forcing work from said hole to said work receiving space.

2. Dielectric heating means comprising a reentrant resonant cavity dimensioned to resonate at a suitable predetermined frequency and comprising an outer shell and an inner hollow tubular reentrant member, portions of said shell and member being conductively secured together, a transverse closing wall for said shell spaced from said tubular member, said tubular member and transverse closing wall being provided with portions forming a nozzle-shaped work-receiving space having a work-discharge nose extending outwardly away from said tubular member, means for exciting the space between said shell and tubular member with high-frequency electrical energy having said predetermined frequency, and means to force work to be heattreated through the inside of said tubular member and through said work-receiving space for discharge through said nose.

3. An invention including that of claim 2 but further characterized by the last said means comprising a plunger slidable in said tubular member.

4. Dielectric heating means comprising a reentrant resonant cavity dimensioned to resonate at a suitable predetermined frequency and comprising an outer shell and an inner reentrant member, said member having a hollow portion to contain work to be heat-treated, portions of said shell and member being conductively secured together and other portions being spaced from each other and shaped to form a work-receiving nozzle having a work-discharge nose, holes communicating between said hollow portion of said member and said work-receiving nozzle, and insulating means filling a part of said spaced portions of said shell and member, for directing work from inside said member to said nozzle, and means for exciting said cavity with electrical energy having said predetermined frequency.

5. An invention including that of claim 4 but further characterized by a pressure-exerting means acting in said hollow portion of said member applying pressure to force work through the nose of said nozzle.

6. An invention including that of claim 5 but further characterized by said pressure-exerting means being a plunger slidably fitting said hollow portion of said member.

7. Dielectric heating means comprising a reentrant cavity dimensioned to resonate at a suitable predetermined frequency and comprising an outer shell and an inner hollow tubular reentrant member spaced therefrom, said shell and member being provided with a nozzle, means for forcing work to be dielectrically heated through said member and then through said nozzle, and means for exciting the space between said shell and member with electrical energy having said predetermined frequency which also permeates said nozzle.

8. Dielectric heating means comprising a reentrant cavity dimensioned to resonate at a suitable predetermined frequency and comprising an outer shell and an inner hollow tubular reentrant member spaced therefrom, a transverse closing wall for said shell, said transverse wall being provided with an outwardly directed nozzle-portion and said tubular member having an inner electrode portion spaced from but extending into said nozzle-portion, and means for exciting said cavity with electrical energy having said predetermined frequency.

9. Dielectric heating means comprising a reentrant cavity dimensioned to resonate at a suitable predetermined frequency and comprising an outer shell and an inner hollow tubular reentrant member spaced therefrom, a transverse closing wall for said shell, said transverse wall being provided with an outwardly directed nozzle-portion, said tubular member having an inner electrode portion spaced from but extending into said nozzle-portion, said electrode portion being circumferentially spaced from said tubular member to provide communication from said member to said nozzle portion, insulating means filling a part of the space between said electrode portion and nozzle-portion, and means for exciting said cavity with electrical energy having said predetermined frequency.

10. Dielectric heating means comprising a reentrant cavity dimensioned to resonate at a suitable predetermined frequency and comprising an outer shell and an inner hollow tubular reentrant member spaced therefrom, a transverse closing wall for said shell, said transverse wall being provided with an outwardly directed nozzle-portion, said tubular member having an inner electrode portion spaced from but extending into said nozzle-portion, and plunger means for forcing work through said inner member and then through the space between said nozzle-portion and electrode portion, and means for exciting said cavity with electrical energy having said predetermined frequency.

11. Dielectric heating means comprising a reentrant cavity dimensioned to resonate at a suitable predetermined frequency and comprising an outer shell and an inner hollow tubular reentrant member spaced therefrom, a transverse closing wall for said shell, said transverse wall being provided with an outwardly directed nozzle-portion, said tubular member having an inner electrode portion spaced from but extending into said nozzle-portion, said electrode portion being circumferentially spaced from said nozzle-portion to provide communication between said member and said nozzle portion, insulating means filling a part of the space between said electrode and nozzle-portions, plunger means for forcing work through said inner member and then through said space, and means for exciting said cavity with electrical energy having said predetermined frequency.

THOMAS W. DAKIN. CARROLL N. WORKS. FITZHUGH W. BOGGS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,386,966 MacMillin Oct. 16, 1945 2,399,930 Keister May 7, 1946 2,443,594 Boettler et a1. June 22, 1948 2,497,670 Hanson et al. Feb. 14, 19b!) FOREIGN PATENTS Number Country Date 580,374 Great Britain Sept. 5, 1946 

